Field of the Invention
The present invention relates to a thin film transistor (TFT) substrate including a thin film transistor as a switching device, a method of manufacturing the same, and a display device including the same.
Description of the Background Art
A thin film transistor substrate (hereinafter also referred to as a “TFT substrate”) is included in an electrooptical device such as a liquid crystal display device (hereinafter also referred to as an “LCD”) as a display device using liquid crystals. A semiconductor device including a TFT is thin and consumes low electric power, and thus has been increasingly applied to flat panel displays.
Examples of LCDs include a simple matrix type LCD and a TFT-LCD that includes a TFT as a switching device for each pixel. The TFT-LCD is superior to the simple matrix type LCD particularly in display quality, and has been widely used in display products such as a mobile computer, a laptop personal computer, and a television.
Typically, the TFT-LCD is provided with a liquid crystal display panel including a TFT substrate and a counter substrate bonded to each other with a constant gap therebetween in which a layer of liquid crystals is injected and sealed. The TFT substrate includes a TFT configured to drive, by switching, pixels in a plurality of pixel regions disposed in an array. The counter substrate includes a color filter (CF). Polarization plates are provided on front and back surface sides of the liquid crystal display panel, and a backlight is provided outside of one of the polarization plates. The color filters provided to the counter substrate are typically in three colors of red (R), green (G), and blue (B) to achieve color display.
For example, Japanese Patent Application Laid-Open No. 2-54217 (1990),
Japanese Patent Application Laid-Open No. 3-237432 (1991), and Japanese Patent Application Laid-Open No. 4-253028 (1992) each discloses, as another configuration different from the above-described configuration, a TFT-LCD configuration in which a color filter is disposed on a TFT substrate, not on a counter substrate. The TFT substrate thus configured is called a CF on-array structure or an on-chip CF layer structure. The CF on-array structure can provide an increased positioning accuracy of a color filter layer disposed on the plurality of pixel regions of a TFT substrate as compared to the conventional configuration in which a TFT substrate is bonded to a counter substrate including a color filter. Thus, the CF on-array structure can reduce color shift, which leads to a high aperture ratio of each pixel, in other words, effective use of the entire pixel as a display region, and as a result, can obtain a high-quality display characteristic with sufficient brightness, for example.
In the switching device of a TFT substrate included in a display device, a channel layer as an active layer of a TFT was made of amorphous silicon (a-Si) in the past. Recently, a TFT including a channel layer made of an oxide semiconductor has been increasingly popularly developed. The oxide semiconductor has a mobility higher than that of the conventionally used amorphous silicon, which leads to a small-sized and high-performance TFT. Technologies of such TFTs are disclosed in, for example, Japanese Patent Application Laid-Open No. 2005-77822, Japanese Patent Application Laid-Open No. 2007-281409, and Kenji Nomura et al., “Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors”, Nature November 2004, Vol. 432, pp. 488 to 492.
A typical oxide semiconductor material has a translucent property with an energy bandgap of 3 eV or larger, and thus it has been conventionally said that the oxide semiconductor material absorbs a small amount of visible light and has a characteristic that is unlikely to change. However, as disclosed in, for example, Chiao-Shun Chuang et al., “Photosensitivity of Amorphous IGZO TFTs for Active-Matrix Flat-Panel Displays”, SID 08 DIGEST, 2008, pp. 1215 to 1218, and Dharam Pal Gosain et al., “Instability of Amorphous Indium Gallium Zinc Oxide Thin Film Transistors under Light Illumination”, Japanese Journal of Applied Physics 2009, Vol. 48, pp. 03B018-1 to 03B018-5, the characteristic of a TFT made of an oxide semiconductor material significantly degrades for light in a short wavelength region, in particular, ultraviolet light having a wavelength of 400 nm or shorter, in some cases.
For example, Japanese Patent Application Laid-Open No. 2007-115902 discloses, as a method for solving the above-described problem, a structure in which a light-shielding layer made of a light-shielding material such as metal or metal silicide is disposed above a TFT including a channel layer made of oxide semiconductor. Japanese Patent Application Laid-Open No. 2014-2392 discloses a CF on-array structure in which an R color filter layer covers all TFTs.